Combination cracking operation



May 26, 1942. H. D. NoLL ETAL COMBINATION GRACKING OPERATION Filed Sept.9, 1939 u w 5 s RLN Y OLA TOR yN NN R E.8 O WOL. n Rw. .mw H Y PatentedMay 26, 1942 UNITED sTATEs PATENT ,OFFICE 2,284,493 l CQMBINATIONCRACKING OPERATION Henry D. Noll, Wenonah, N. J., and Otis L. Branson,Beaumont,

Vacuum Oil Company,

Tex., assignors to Socony- Incorporated. New York,

N. Y., a corporation of New York I Application september 9, 1939, seraiNo. 294,110

(ci. '19e-5s) 1 Claim.

vaffords more eiiicient heat utilization.

In operating a conversion system in accordance with a preferredembodiment of our invention, a reduced crude heated by exchange is fedinto an evaporator tower which receives all of the hot streams of thesystem. An overhead from the evaporator tower passes to a bubble tower.The residue from the bottom of the evaporator towerpasses to a fuelflash tower. A heavy gas oil fraction withdrawn from near the top of theevaporator tower and a light gas oil withdrawn from the bottom of thebubble tower are passed through separate coils of a cracking furnace andthen passed together through'external transfer line heat exchangers.From the transfer line exchangers the cracked gas oils are passed intothebottom of the evaporator tower. A heavy naphtha fraction heated byexchange with the cracked gas oils in the transfer line exchangers ispassed into a reforming furnace. From the reforming furnace the reformednaphtha passes through additional external transfer line heat exchangersand then is passed into the bottom of the evaporator tower.

A reduced crude fraction is withdrawn from near the middle of theevaporator tower and passed through the transfer line exchangers of thereformer furnace in indirect heat exchange with the reformed heavynaphtha passing therethrough and then injected into the reformed heavynaphtha line going into the exchangers and thereby iiowed through thetransfer line exchangerswith the reformed heavy naphtha and then intothe bottom of the evaporator tower.

For a more complete understanding of the present invention referencewill be had to the accompanying drawing wherein the figure is adiagrammatic drawing of a preferred conversion system employing ourinvention. y

In the drawing reduced crude is fed through line I, containingexchangers 2 and 3 of the bubble tower 4 and evaporator tower 5,respectively, intovthe middle of evaporator tower 5. Overhead distillatefrom the evaporator tower line 6 through exchanger Sinto bubble towe 4.Residue from the evaporator tower is passed through line 'I to fuelflash tower 9, the ash distillate from which is returned to the top ofevaporator tower 5 through line 9.

Overhead distillate from bubble tower 4 passes through line I9containing exchanger 2 and cooler Illa to a separator II. Uncondensedgas from chamber II passes tofuel and liquid is sent principally throughline I2 to a stabilizing unit (not shown) with a portion being returnedthrough line I3 to the top of tower 4 as reflux.

A heavy gas oil fraction withdrawn from tower 5 through line I4,containing vapor separator I5 from which vapors are returned to tower 5through line I6, is sent to one set of coils in the light and heavy gasoil cracker I1. Light gas f oil withdrawn from the bottom of bubbletower 4 is sent through line I8 to the other set of coils lin crackerI'I. The cracked heavy and light gas oil withdrawn from cracker I1 arefed together through line I9 into the external transfer line exchangers20. and from exchangers 29, the gas oils pass through line 2| to thebottom of tower .transfer line exchangers 20 wherein it is heated up byindirect heat exchange with the cracked light and heavy gas oil passingtherethrough. The heavy naphtha passes from exchangers 20 to the coilsof the reformer furnace 23 and is subjected reforming therein. The heavynaphtha withdrawn from reformer 23 through line 24 is fed into transferline exchangers 25. and from exchangers 25, the heavy naphtha passesthrough line 26 into the bottom of tower 5.

A reduced crude fraction withdrawn from near the middle of tower 5 ispassed by line 21, containing gas separator 28 from which vapors arepassed back to tower 5 through line 29, to transfer line exchangers 25.VThe reduced crude fraction is passed through exchangers 25 and heatedup to approximately viscosity breaking temperature therein by indirectheat exchange and the thus heated reduced crude fraction is theninjected into the reformed heavy naphtha line 24 leaving reformerfurnace 23 and fed along with the reformed heavy naphtha back throughexchangers 25 and viscosity broken therein and then passed along withthe reformed heavy rsiaphtha through line 26 into the bottom of tower Animportant feature of the present invention is the dual function of thetransfer line exchangers of the reformer furnace, i. e., the function ofserving as an external unflred heating chamber for heating the reducedcrude fraction up to approximately viscosity-breaking temperaturesandthe further function of se1 ving as an external unfiredviscosity-breaking unit'for the reduced crude fraction. |Thus, as seenabove. these exchangers heat the reduced crude fraction up totemperature by heat exchange and then viscosity breaks the heatedreduced crude as it is fed back through the exchangers in admixture withreformed heavy naphtha. It is obvious, of course, that use may be madeof this important feature of our invention in conversion systems otherthan the preferred embodiment described herein.

The construction and operation of the reformer transfer line exchangersmust be such as to aiiord proper contacting time of the oils passingtherethrough at the required temperatures for viscosity breaking of thereduced crude.

Ii it is required to control the volume of injected oil over wideranges, this may be accomplished by introduction of reflux from line 9into tower at points A and B. The introduction at A will aifectthegravity of the fuel eliminated and tend to produce a lighter cycle inthe in- Jection flow stock passing through line 21 to exchangers 25.With increased reflux at B, the

rate of injection iiow will increase, whereas a decrease of refluxproduces less injection ow material.

Since the temperatures and contact times may vary inversely over anappreciable range and may be somewhat diiferent depending on theparticular oils and fractions being treated and the amount of viscositybreaking desired, it would be diicult to specify any denite values forthese conditions which would suit all operations. However, as suchconditions as these are often encountered by workers in the art, it isbelieved little trouble will be had in adapting the present invention toany particular operation.

On the other hand, if precise determination of soaking time is desired,it may be made as shown in the following sample calculation fordetermining proper soaking time in exchangers 25 to give a 10% crack perpass on black oil under certain given conditions.

Sample calculation Soaking time can be calculated from the averagevolume of the oil vapors in the soaking zone and the volume of thesoaking zone. The volume of the soaking zone is calculated from the coldvolume of oil feed in barrels per day and the soaking factor.

The soaking factor is dened as .Kr/Kaum r. (coil vol. in cu.ft./bbl./day). Kir/Kano is the ratio of the cracking reaction velocityconstant at a temperature T to the value at 800 F. In general, thesoaking factor is evaluated by graphical integration of a plot ofKr/Kauo vs cu. ft. /bbL/day.

Soaking volume-Given that the oil vapors enter exchanger or soaking zoneat 936 F. and leave at 850. By assuming the temperature fallslogarithmically intermediate temperatures can be found and, hence, KT/Kaoo. In the following table the first column gives the fraction ofthe total exchanger volume, the second column the correspondingtemperatures, the third column the value of Kr/Kaoo, and in the fourthcolumn the integration by Simpsons rule. The total feed to the soakingzone is 7000 B/D of heavy naphtha and 8000 B/D of black oil, a total of15000 B/D.

Using V for exchanger volume and h for Width of strip in Simpsons ruleFraction of Integration exchanger Tgllgr' K r/ Km h o 25X vo umo 15000F. 0. 00 936 39. 0 39. 0 0. 25 914 22. 7 90. 8 0. 50 893 13. 3 26. 6 0.873 7. 5' 30. 0 1 o0 85o l. 2 4. 2

Then by Simpsons rule for integration the soaking factor is-}X190.6X0.25X V-=0.00106V From data for Magnolia operation, it is knownthat the soaking factor should be numerically equal-to 0.065 to give a10% C/P on the black oil. Hencee 0.065,=0.00106 V and V=61.4 cu. ft.

Oil vapor volume-Because of the extreme conditions imposed on the oilvapor, its volume cannot be calculated from the ideal gas law withoutusing a deviation factor. The average temperaturev is about 890% F. andthe pressure about 600 lbs./sq. in. absolute in the soaking zone.

Black oil for injection- Given that the A. S. T;TM.-50% point is 850 F.,the API gravity is 20 and the U. 0. P. characterization factor is 11.5,it can be determined that the average molecular weight is 370 and thatthe critical temperature and pressure are 1160 F. and 175 lbs./sq. in.respectively. The deviation factor is then determined to be 0.517.

Reformed gasolina- Given the gravity as 58.3 API and the temperature atwhich 10, 30, 50, 70, and is evaporated in the A. S. T. M. distillationas being 157, 223, 275, 323, 375 F. respectively. 'Ihese data determinethe U. O. P. characterization factor as being 12.0, the molecular weightat 116.5, and the critical temperature and pressure to be 595 F. and 398lbs./sq. in. The deviation factor is then 0.755.

Gas.-By assuming a molecular weight of 33, the critical temperature andpressure of the gas will be about 104 F. and 691 iba/sq. in. Thedeviation factor is 0.993.

Mols and liquid volumes of materials.-Black oil.

At the average condition about 5% converted to gasoline, hence 80000.95=7600 B/D of black oil (20 API equivalent to 7.778 iba/gal.)

'Ihe number of mois per minute is Gasoline.

The number of mols per minute is Gas. Assume same number of mois formedas of gasoline.

Vapor volumes.-The corrected gas law is cu. lft. 1bs./sq. in. R' s 1071deg. Rankine Black oil- 24.12X 4.65X0.517= l58.0 cu. ft. Gasoline24.12X11.5QXO.755l-209.3 cu. ft. Gas 24.1 2X11.50X0.993=2'15.2 cu. ft.

Total volume 542.5 cu. ft. Soaking time=61f4i =0.1l3 minutes--or 6.8sec.

-problems and by adjustments arrive at satisfactory operatingconditions, as is often done in cracking practice.

While the transfer line exchangers of the reformer furnace have beendescribed and shown as' the viscosity-breaking unit, it is to beunderstood that other transfer line exchangers for an oil ofsuiiiciently high temperature might be used such as the transfer lineexchangers 20 of the light and heavy gas oil cracker I1. Likewise thetransfer line exchangers of one oil line might be used to preheat thereduced crude fraction and the transfer line exchangers of another oilline used to viscosity break the fraction. Thus the reduced crudefraction in the preferred embodiment shown might be preheated inexchangers 20 of cracker I1 and viscosity broken in exchangers 25 ofreformer 23. It is to be understood further that the present inventionis not restricted to the exact cracking and reforming systern hereinshown but, as is obvious, may be applied to such systems in general.

The temperature to which the reduced crude fraction should be preheatedbefore being injected into the reformed naphtha should be suiiicientlyhigh that when combined with reformed heavy naphtha the mixture willhave an eicient viscosity breaking temperature. This preheatedtemperature of the reduced crude fraction will depend therefore on thetemperature of the reformed naphtha or other stock and the relativeproportions of the oils being mixed. As a matter of practice it will befound that the reduced crude fraction usually should be preheated tosubstantially the viscosity breaking temperature before being injectedinto the reformed heavy naphtha. It will be noted that by the presentinvention wherein the reduced crude fraction is preheated and viscositybroken in an external unred zone, the fraction is never subjected to theconditions of directly fired tubes with the attendant danger of cokedeposition.

In order to furtherillustrate'the present in;-

vention we will describe a specific operation using a mid-continentreduced crude, however, it is to be understood the invention is not tobe limited by this example as there may be variations therefrom withoutdeparting from the scope of the invention.

Reduced crude from a mid-continent stock is fed at the rate of 6000bbl/day through line I containing exchangers 2, and 3 into evaporator-tower 5. Tower 5 is -maintained under 250 lbs/sq. in, pressure andv hasa temperature of 720" F. at the top' and 800 F. at the lbottom. Vaporfrom the top of tower 5 passes through line 6 containing exchanger 3into bubble tower 4. Bubble tower 4 is maintained'under a pressure of240 lbs/sq. in. and has a top temperature of 420 F. and a bottomtemperature of 600 F. Residue from tower 5 passes through line l to fuelash tower'8. the flash distillate from which is returned to the top oftower 5 and the residue'yields 2730 bbls./day of fuel oil.

Overhead vapor from bubble tower 4- passes througnline I0 containingexchanger 2 and cooler Illa to vapor separator II which is maintainedunder a pressure of 225#/sq. in. and at a temperature of F. The gaseousproduct removed from the top of separator I I goes to fuel, while theresidue is'withdrawn at the bottom, a portion of which is returnedthrough line I3 as a reflux to the top of bubble tower 4, and the re-.;.mainder of which is sent through line I2 to a stabilizing unit (notshown).

Light gas oil recycle withdrawn through line I8 at the ,bottom of bubbletower 4 is passed at the rate of 10,000 bbls./day to one set of coils incracker I1. Heavy gas oil withdrawn through line I4 from near the top oftower 5 `passes into vvapor separator I5 maintained at set of coils incracker I1. The cracked light gas oil at a temperature of 980 F. and thecracked heavy gas oil at a temperature of 925 F. are combined in line I6and fed together under a pressure of '150#/sq. in. into exchangers 20and then through line 2l into the bottom of tower 5. The cracked gasoils remain in exchangers 20 for about 42 seconds, in which it issubjected to soaking for about 21 seconds since only about one-half ofthe exchanger volume is above 800 F.

Heavy naptha at the rate of about 700 bbls./ day is fed through line 22containing exchanger 20, wherein it is heated up by exchange with thecracked gas oil, to reformer furnace 23, the reformed heavy napthaleaves the reformer furnace at a temperature of around 1000 F. throughline 24 where it is mixed with a reduced crude fraction and the mixturefed-into exchangers 25 at a temperature of about 936 F. The mixtureremains in exchangers 25 for about 7 seconds during which time viscositybreaking of the reduced crude fraction admixed therewith occurs. Themixture leaves exchanger 25 at a temperature of about 850 F. and ispassed through line 26 into the bottom of tower 5.

The reduced crude fraction which is to be combined with the reformedheavy naphtha is withdrawn from near the middle of tower 5 through line21 and passed into vapor separator 2B which is maintained at '750 F.Vapors fromseparator 28 are returned to tower 5 through line 29. Theliquid from separator 28 passes through line 21 at the rate of 800bbls./day to exchangers 25 and passes through exchangers 25 wherein itis heated up by exchange and then injected into an overhead vapor fromsaid tower is passed to a bubble tower, light gas oil from said bubbletower and heavy gas oil from said evaporator tower are sent throughseparate heated coils under selective cracking conditions therefor andthe cracked oils therefrom are passed to said evaporator tower, andwherein preheated naphtha is passed through a heated reforming coilunder reforming conditions to effect substantial reforming thereof, theimprovement which cornprises passing a reduced crude withdrawn from saidevaporator tower through an external uniired soaking zone to preheatsame to a predetermined temperature, introducing such preheated reducedcrude into substantially reformed naphtha withdrawn from said heatedreforming coil and regulating the predetermined temperature and amountof the preheated reduced crude so introduced that the resultanttemperature of the mixture is at an emcient viscosity-breakingtemperature for the reduced crude. passing the mixture through saidunred soaking zone under viscosity-breaking conditions, in indirect heatexchange with said reduced-crude passing therethrough alone, to eifect asubstantial viscositybreaking of the reduced crude which is mixed withsaid naphtha, and then passing the mixture from said soaking zone tosaid evaporator tower for separation into vaporous and liquid products.

HENRY D. NOLL. OTIS L. BRANBON.

GERTIFIGATE oF conmzcuou. Pnentno. 2,231hh95. y nay 26, 19m. HENRY n.Nom., nu..

I't 1a herebycerttied that error appear; 1n the printed specification'of the above nu'mbered. p atent requiringcorrecton as follows: Page 1,aecond column,A line 50, after "subjected" insert --to--g page 2I secondc olumn, line 51 for *890%* read .890f; page 5, second column, line 57,for *line 16" read --line 19"; line 1lb., fo;` 'F700' read 7O00; line66, for *80o* read -'8oo; and that the said Letters Patent should beread with this correction therein that the seme may conform to therecord of the case 1h the Patent office.

Signed end sealedlths 29th day of September, A. D. 1%2.

Henry Van Aredale,

(Seal) Acting Commissioner of Patents.

